This invention concerns new polyorganosiloxane compositions containing fired silica which give silicone elastomers by thermal vulcanization such as hot air vulcanization which elastomers have good resiliency, good heat resistance in a closed system, low compression set and improved electrical properties and a high modulus.
As reinforcing materials for silicone rubber there are two different series of silica; one is a dry type silica obtained with heat decomposition of halogenated silicon in a high temperature oxygen-hydrogen flame, and the other one is a wet type silica obtained with decomposition of a silicate by acid in the presence of water. The former is called fumed silica and the latter is called precipitated silica, and varieties of these products are extensively used. Other than these, there is silica aerogel as a variation of the wet type silica obtained by adding heat above the critical temperature of the organic solvent and pressure above the critical pressure of the organic solvent to an organogel by replacing the moisture in the silica hydrogel by an organic solvent such as alcohol, in which the silica hydrogel has been obtained by decomposing silicate with acid in the presence of water. Although fumed silica provides silicone rubber with excellent physical and electrical properties except for resiliency and compression set, it requires equipment of special construction and materials, and requires stringent control over reaction conditions because it involves decomposing thermally halogenated silicon at a high temperature. In addition, it is not economically advantageous because it requires treatment of a halogenated hydrogen as a by-product for environmental reasons. As to silicone rubber obtained from the composition using fumed silica as a reinforcing agent, the resiliency is so low and compression set is so high that these are shortcomings of such silicone rubber. Precipitated silica is economically advantageous, because there is no necessity for special equipment in manufacturing, control over reaction and treatment of halogenated hydrogen as a by-product. However, in the case that precipitated silica is used for reinforcement for silicone rubber there are disadvantages such as foaming during the process of hot air vulcanization under normal pressure, low direct current resistance of the silicone rubber obtained and great dielectric loss. Further, despite the complex manufacturing process of silica aerogel, the afore-mentioned disadvantages have never been greatly improved by the use of silica aerogel.
For the purpose of solving the foaming problem of silicone rubber with the use of precipitated silica and improving its electrical properties, various improvements have been made. However, there have been no satisfactory results in the use of precipitated silica alone. Even though the surface of precipitated silica is treated with organosilicon compounds such as organochlorosilanes, organosilazanes, and polyorganosiloxanes, the foaming phenomenon during the process of hot air vulcanization under normal pressure is observed. Accordingly, at present, electrical properties are improved and hot air vulcanization under normal pressure is made possible by means of the use of the expensive fumed silica.
Polyorganosiloxane compositions from the viewpoint of a bridging (vulcanizing) mechanism, can roughly be classified into two categories. One is a conventional method that performs crosslinking or so-called peroxide vulcanization, in which radicals that were formed in the decomposition of organic peroxides through the process of heating polyorganosiloxane with a high degree of polymerization at a high temperature in the presence of an organic peroxide, attack organic groups of the polyorganosiloxane and take hydrogen out and also bring about electron transfer of vinyl groups and so forth. However, the existence of decomposition products of such organic peroxides, particularly of aromatic acyl peroxides that are generally used as a powerful vulcanizers in vulcanized silicone rubber gives an unfavorable influence to silicone rubber. Taking the most popular example of benzoyl peroxide, when benzoic acid formed in the decomposition exists in silicone rubber, it breaks up siloxane chains at high temperatures particularly in the presence of hot steam; consequently it hinders steam resistance and heat resistance in a closed system. Further, various by-products of such peroxides contain health hazardous residues such that there are restrictions in the selection of organic peroxides used as vulcanizers, when the silicone rubber is used for transplanting in the human body, for other medical purposes, food-handling and so forth.
The silicone composition of the present invention is the platinum-cure or addition reaction type silicone rubber in which crosslinking is performed by a hydrosilylation reaction of a polyorganohydrogensiloxane to vinyl groups of a polyorganosiloxane in the presence of small amounts of platinum as a catalyst. This kind of reaction has been used from the earliest times as a vulcanizing mechanism for RTV type silicone rubber which has polyorganosiloxane containing vinyl groups with relatively low degree of polymerization and have fluidity as a base polymer. However, there has been delayed the application of the same reaction to an area in which the degree of polymerization of the base polymer is relatively high. This is attributable to the fact that the stability of such compositions after adding the catalyst is so poor that roll milling operations and other manufacturing processes are impossible to undertake because a complex such as chloroplatinic acid with alcohol, olefin or vinyl containing low polymerized polysiloxane makes the hydrosilyl reaction take place gradually even at room temperatures, and in such a base polymer which has a higher polymerization degree, a slight hydrosilylation gives gelation.
Many methods to control reaction at near room temperature have been reviewed to improve these shortcomings. For example, in the Japan Patent Publication 31476/69, there is disclosed the use as an inhibitor of an organic compound possessing an acetylene bond in which at the alphaposition a functional group containing a nitrogen, phosphorus or sulfur atom exists as well. In the Japan Pat. Publication No. 19193/67 there is disclosed the change of platinum to a divalent platinum complex with phosphorus, so as to inhibit the reaction.
In the case that the inhibitor described in the Japan Pat. Publication No. 31476/69 is used, it has an advantage in that an arbitrary reaction-speed can be selected by means of certain amounts of the inhibitor. However, as a matter of practice, it is difficult to control such an amount, and if any excess amount is added, it may cause poor vulcanization. The inhibitor also cannot play its proper functions well if it evaporates or has an unfavorable influence on the silicone rubber obtained. Furthermore, in the case of the inhibitor described in the Japan Patent Publication No. 19193/67, inhibiting effects can be attained to some extent; however, sufficient results cannot be expected even in this method. Moreover, which such an inhibitor is used, the vulcanization does not proceed even with heating in a closed condition. Moreover, such a method has the common disadvantage that the chlorine contained in a platinum compound has adverse effects on silicone rubber. For the purpose of removing these disadvantages the use of the zero valent platinum phosphorus complex, disclosed in the Japan patent application No. 139853/76, is suggested and the stability of the polyorganosiloxane at room temperature is improved. Furthermore, by adding a stannous salt to this zero valent platinum phosphorus complex it improves the stability of the complex itself and the suggestion is made that it improves the workability of the milling and molding of the silicone rubber. (See Japan patent application No. 61924/77).
Although the utility of addition reaction type silicone rubber has been considerably enlarged by these suggestions, there still exist limitations due to the aforementioned filler; that is; difficulties such as low resiliency, the great compression set and the unfavorable economic feasibility of silicone rubber with fumed silica. Further, there is the poor electrical properties and the foaming problem in the process of hot air vulcanization under normal pressure when precipitated silica is used or aerogel silica. These problems are similar to those observed in the case of peroxide vulcanization.